1. Field of the Invention
The present invention relates to an optical line termination, a passive optical network system and a method of dynamically controlling an upstream band.
2. Description of the Related Prior Art
A conventional passive optical network (hereinafter referred to as a PON) system comprises an optical line termination (OLT), a plurality of optical network terminations (ONTs) and a star coupler for coupling the OLT and the ONTs. For communications between the OLT and the ONTs, an asynchronous transfer mode (ATM) is used. For downstream communications from the OLT to the ONTs, a time division multiplex (TDM) communication is utilized. All ATM cells to be a transmitted to the ONTs are sent out to the ONTs. Each ONT selects an ATM cell transmitted to itself, and ignores other ATM cells. For upstream communications from the ONTs to the OLT, a time division multiple access (TDMA) communication is utilized. A cell transmitted from each ONT is allocated fixedly and previously in a frame of a TDMA format by an instruction from a network management system (NMS) on the OLT side. In other words, it is impossible to dynamically control the upstream band of ATM-PON. ITU-T recommendation G.983.1 (BROADBAND OPTICAL ACCESS SYSTEMS BASED ON PASSIVE OPTICAL NETWORKS (PON), October 1998) touches on an operation and maintenance (OAM) cell which performs a band change request, in order to dynamically perform a upstream cell allocation. However, this recommendation does not show a control method concretely.
The object of the present invention is to provide a system and an OLT, which are capable of dynamically changing an upstream band in ATM-PON.
The OLT of the present invention comprises a dynamic cell allocation section and a control section. The dynamic cell allocation section sets an upstream band based on existence of an alarm and a type of the alarm which are transmitted by each ONT. The control section controls the communication with the ONTs. The OLT is capable of comprising a message transmitter-receiver section and a cell allocation section for outputting a cell allocation signal. The dynamic cell allocation section is capable of comprising a circuit for detecting existence of an alarm and a type of the alarm, which are transmitted from the ONT, and for outputting a signal which sets an upstream band of the ONT based at least on the detection result of this time and the last detection result. The dynamic cell allocation section is capable of comprising a computing section for computing and holding a total optical network termination cell allocation in a cell allocation cycle based on an upstream cell rate, which is set for each ONT, and for outputting a computing result to the cell allocation section.
A PON system of the present invention comprises the ONTs and the above mentioned OLT. The ONT comprises a control section, a message transmitter-receiver section, a buffer section, and a buffer control section. The buffer section holds data to be transmitted to the OLT, and outputs a threshold exceeding alarm when a quantity of data is epual to a first threshold or more. The buffer section outputs an empty alarm when the quantity of data is equal to a second threshold smaller than the first threshold or less. The buffer control section transmits the alarm to the message transmitter-receiver section, and controls the buffer section.
A method of dynamically controlling an upstream band of the present invention comprises the steps of setting plural types of upstream bands for each ONT; computing a cell allocation corresponding to the bands for a frame, thus holding a computing result; detecting existence of an alarm and a type of the alarm, which are transmitted by each optical network termination; referring at least to a detection result of this time and the last detection result, thus setting an upstream band of the ONT based on conditions previously set; computing a cell allocation position for each of the ONT in accordance with a band specification; and transmitting the cell allocation position computed for each ONT to each ONT.